1. Field of the Invention
The present invention relates to a logic valve of a type that controls pressure in a spring chamber to control the volumetric flow of oil to a hydraulic valve used for construction machines and similar machines.
2. Description of the Prior Art
The applicant of the present invention has previously applied for a patent for a pressure compensating type logic valve with the specification and the drawings of Japanese Patent Application No. 319169/1988, which is a prior invention of the present invention.
As shown in FIGS. 3 and 4 the referenced pressure compensating type logic valve comprises a sleeve 12 fitted inside a housing 11. A logic valve poppet body 13 is slidably fitted in sleeve 12. Logic valve poppet body 13 includes, at one side thereof, a loading pressure inlet portion P.sub.IN, which is used as the flow controlling portion of a hydraulic pressure control valve and through which loading pressure is introduced. At the other side, logic valve spring chamber 14 contains a spring 15. Spring 15 urges logic valve poppet body 13 against a seat 16 connected to a drain port T.sub.1. Pilot spools 18 and 19 are installed in a separate housing 21. Pressure from loading pressure inlet port P.sub.IN acts upon an end surface of a pilot spool 20 through an orifice 29. Pilot spool 20 is installed in housing 22 to control fluid conducted from loading pressure inlet port P.sub.IN to spring chamber 14 at the other side through an orifice 17. Pressure from a spring 23 is applied to pilot spool 19 and a pilot pressure in the direction against spring 23 is applied from a valve controlled from outside to a pilot pressure chamber 24 through an external pilot pressure inlet port P.sub.1. Pressure is conducted from logic valve spring chamber 14 into a pressure chamber 27 through a detection path 25 and an orifice 26. The pressure in pressure chamber 27 is applied to pilot spool 18 in the axial direction opposite to the force of spring 23, and loading pressure conducted from loading pressure inlet portion P.sub.IN into the other pressure chamber 30 through the other detection path 28 and orifice 29 is applied upon pilot spool 20 in the same axial direction as that of the spring force thereof. Oil in spring chamber 14 for pressure is bled into drain port T.sub.2 through bleed groove 32 and seat 31 when the conical portion of spool 19 has been detached from seat 31.
In case differential pressure has occurred between loading pressure inlet port P.sub.IN and spring chamber 14, such differential pressure works upon pilot spools 18 and 20 through the two detection paths 25 and 28 respectively and becomes balanced, with respect to the force of pilot spool 19 in the axial direction, with pilot pressure introduced from the externally controlled valve through pilot pressure inlet port P.sub.1 as well as with the force of spring 23. By taking advantage of such balance, it is possible to control the stroke of the logic valve in a stable condition without being under the influence of the absolute value of loading pressure of loading pressure inlet port P.sub.IN.
The above prior invention requires spools 18, 19 and 20 to control strokes of logic valve poppet body 13, and housings 21 and 22 to contain these spools separately from logic valve poppet body 13. Therefore the valve of the prior invention presents the problem that it is more complex and larger than necessary.
Further, it has been observed that, when pressure is applied at loading pressure inlet port P.sub.IN, leakage occurs at four locations: leak Q.sub.1 at the diametrical space between sleeve 12 and logic valve poppet body 13; leak Q.sub.2 at the diametrical space between spool 20 and housing 22, into which oil is conducted from loading pressure inlet port P.sub.IN through path 28, orifice 29, etc.; and leaks Q.sub.3 and Q.sub.4 at the diametrical space between pilot spools 18 and 19 and housing 21, into which internal pressure in logic valve spring chamber 14 is conducted. Since it is desirable to keep the amount of leakage as small as possible, the number of locations of possible leakage also should be kept as small as possible.
An object of the present invention is to make a logic valve more compact with less leakage.